1. Field of the Invention
2. Background of the Invention
Coal is a large energy source. It has been mined from the earth for many years. Deposits of coal beneath the ground surface are positioned in generally horizontal coal seams and include substantial quantities of methane gas entrained in the coal deposits. In underground coal mining, methane gas poses a significant safety risk to the miners. In the past, the methane gas entrained in the coal deposits was simply liberated from the coal, mixed with air in the mine which diluted it to a safe concentration, and the mixture was ventilated to the outside environment. The methane was simply dissipated into the environment and provided no meaningful resource. However, in recent years, this entrained methane gas has been sold commercially as an energy source, typically as a driving source for energy-producing equipment, such as a generator or added to natural gas pipelines.
Utilizing the gas as an energy source requires that the gas be extracted in a concentrated state and captured. Extracting methane from the coal seams in a concentrated state has been achieved by drilling boreholes generally horizontal into the coal seam that extend several thousands of feet.
During and after the methane drilling process, dewatering must occur. Since coal seams have a significant amount of subterranean water associated with them, this water must be drained from the coal seam in order to produce the methane. Further, during the drilling process, water is used at the drilling tip, creating a slurry of drill cuttings, which also must be removed from the borehole. Water and drill cuttings can block the migration of gas through the coal seam to the borehole and therefore must be removed to permit degasification. Additionally, some of the water used in the drilling process can be forced under pressure into the coal seam, further saturating the gas reservoir, which impedes the migration of gas to the borehole. Therefore, dewatering must occur both during the drilling process and after drilling has been completed. Two methods of dewatering during drilling horizontal portions of the borehole in the coal seam are 1) use a combination of concentric casings installed in the vertical and curve portions of the bore in which compressed air would be forced under pressure between the annular space of the casings to air lift the drill effluent or waste to the surface between the annular space of the inner casing and the outside of the drill string, or 2) use one casing and a compressed air tubing run and cemented in the vertical and curve portions of the bore in which compressed air is forced in the tubing to lift the drill effluent to the surface between the annular spacing of the inside of the casing and the outside of the drill string. After drilling in the coal seam is completed, and the drill string is retrieved to the bottom of the curve portion, a rathole or sumphole would be drilled through the coal seam and into the floor below the coal seam to install a dewatering device(s) in the rathole where water and fines will temporarily collect until the dewatering device pumps them to the surface.
Long, generally horizontal boreholes that remain in the coal seam are the most effective manner to extract and capture the gas entrained in the coal seam providing the suitable reservoir and material strength characteristics exist in the coal seam. Horizontal and generally horizontal holes can be effective in a suitable coal seam because they remain in contact with the gas reservoir (the coal bed) for long distances. Typical generally horizontal directional boreholes are drilled from inside the coalmine, which create several safety concerns, require the use of specialized equipment, and usually have limited borehole productive life. Transportation of gas in a pipeline, inside an underground coalmine, requires considerable maintenance and safety inspections due to the explosive nature of gas. Although directionally drilled inseam holes can reach several thousand feet, the holes do not always provide complete degasification before the coal seam is mined. Oftentimes the coal mining operator must mine the coal in an area before complete degasification has been achieved. Therefore, a method to maximize coal bed gas recovery while reducing the safety risks to the coal operators is desirable. Furthermore, in areas not associated with current or future coal mining, the dewatering methods described herein, both during and after drilling will maximize coal seam methane recovery.
Long, generally horizontal boreholes drilled from inside the coalmine are relatively easy to dewater due to the ability of the gas to purge water from the borehole because the boreholes are generally level with the end of the borehole, therefore, the gas does not have to overcome substantial hydraulic head to purge the water from the borehole.
It is an object of the present invention to overcome the deficiencies inherent in the prior art. It is another object of the present invention to provide a method for producing gas from a coal seam yielding increased operator safety. It is another object of the present invention to provide a method for producing gas from a coal seam while simultaneously producing in-situ water, water induced in the coal seam while drilling by the drill mechanism and drill cuttings during the directional drilling operation. It is yet another object of the present invention to provide a method of simultaneously producing gas and water from a coal seam after drilling is complete to allow increased gas exploitation of a coalfield.